1. Field of the Invention
The present invention relates to speech communications between mobile terminals of a mobile communications system, and more particularly to communications through an IP network present in the communications route.
2. Description of the Related Art
Mobile communications systems employ a speech coding process having a lower bit rate and a high band compression ratio in view of the frequency utilization efficiency in wireless intervals. When mobile terminals belonging to different mobile communications systems communicate with each other, a communications path is established through gateways which interconnect the two mobile communications systems. Even if the mobile communications systems employ the same speech coding process, a signal passing through a transit network is converted by a general-purpose speech coding process such as 64 kPCM unless the gateways and the transit network are compatible with the speech coding process of the mobile communications systems.
FIG. 1 of the accompanying drawings shows a communications path established for communications between conventional mobile communications systems. Mobile switching center (MSC) 903 and mobile switching center 907 belong respectively to different mobile communications systems, and are connected to each other by transit network 906 between the mobile communications systems. A communication path is established between mobile terminal (MT) 901 belonging to one of the mobile communications systems and mobile terminal 910 belonging to the other mobile communications system. Common channel signaling (CCS) of SS7 (Signaling System number 7) is employed between mobile switching centers 903, 907 and mobile terminal 910, and a control signal is separated from a user signal.
Mobile switching center 903 has transcoder 904 and controller 905. Mobile switching center 907 has transcoder 909 and controller 908. Mobile terminal 901 has coder/decoder (codec) 902. Mobile terminal 910 has coder/decoder (codec) 911.
The two mobile communications systems employ the same speech coding process. Therefore, codec 902 and codec 911 encode and decode speech signals according to the same process. Alternatively, codec 902 and codec 911 may have a plurality of speech coding processes and select any one of those speech coding processes. In such a case, codec 902 and codec 911 may have at least one common speech coding process among those plural speech coding processes.
Transcoders 904, 909 convert signals between different coding processes. Transcoders 904, 909 provide a general-purpose speech coding process, such as 64 kPCM, toward transit network 906. Transcoders 904, 909 also provide a speech coding process having a high compression ratio, which the mobile terminals have, toward the mobile terminals. Transcoders 904, 909 convert signals between the speech coding process having a high compression ratio and the general-purpose speech coding process. Usually, one mobile switching center has a plurality of transcoders. When calls are made between the mobile communications system to which the mobile switching center belongs and another mobile communications system, the transcoders are assigned to those calls. The speech coding process specific to the mobile communications system is used between the mobile terminals and the mobile communications system to which the mobile terminals belong, and the speech coding process specific to the transit network is used between the mobile switching centers with the transit network interposed therebetween.
Controllers 905, 908 establish calls, establish communication paths, and assign transcoders to calls.
In FIG. 1, a call is established between mobile terminals 901 and mobile terminal 910. The call is made through a communication path which extends through coder/decoder 902 of mobile terminal 901, transcoder 904 of mobile switching center 903, transcoder 909 of mobile switching center 907, and codec 911 of mobile terminal 910. Speech signals between codec 902 and transcoder 904 and signals between codec 911 and transcoder 909 are processed by the speech coding system having a high compression ratio. Speech signals between transcoders 904, 911 are processed by the general-purpose speech coding system.
Therefore, speech signals are converted twice between different speech coding processes for communications between mobile terminals belonging to different mobile communications systems. Such a connection is referred to as a tandem connection. The tandem connection suffers large speech quality deterioration because signals according to the speech coding system having a high compression ratio are compressed and expanded twice. In order to improve the speech quality, the tandem connection may not be employed, and the codecs of the mobile terminals may directly be associated with each other.
According to 3GPP (3rd Generation Partnership Project), there is proposed TFO (Tandem Free Operation) for directly associating the codecs of mobile terminals with each other using an in-band control signal. According to the TFO, mobile switching center 903 and mobile switching center 907 insert bits for controlling the coding process into in-band user signals in communications to negotiate with each other. If possible, mobile switching center 903 and mobile switching center 907 bypass transcoder 904 and transcoder 909, respectively. In this manner, codec 902 of mobile terminal 901 and codec 911 of mobile terminal 910 are directly associated with each other. Such a connection is referred to as a bypass connection. According to the TFO, the configuration may switch from the bypass connection back to the tandem connection.
The 3GPP also proposes TrFO (Transcoder Free Operation) for directly associating the codecs of mobile terminals with each other using an out-band control signal. According to the TrFO, a control signal of the coding process is defined as an out-band signal of SS7, i.e., a control signal separated from a user signal, and is used for controlling the bypass connection and the tandem connection.
The above conventional arrangement suffers the following problems: The control process using the in-band control signal is made possible after communications between mobile terminal 901 and mobile terminal 910 have been established. According to the TFO, immediately after a call is made, the transcoders of mobile terminals are used, and the bypass connection is established using the in-band control signal. According to the TFO, therefore, communications of good speech quality based on the bypass connection are not possible immediately after the call is started. The TFO is also problematic in that since control bits are inserted into the user signal, a portion of the user signal is removed when the control bits are transmitted, resulting in a reduction in the communications quality. According to the TFO, furthermore, transit network 906 is limited to an STM network based on PCM, and VoIP cannot be realized using an IP network as transit network 906. According to the TrFO, since the control signal is separated from the user signal, the user signal is not removed upon switching between the tandem connection and the bypass connection, and the tandem connection or the bypass connection can be selected when a call is established. According to the TrFO, as with the TFO, transit network 906 is limited to an STM network based on PCM, and VoIP cannot be realized using an IP network as transit network 906.